As explained in the aforementioned copending application, it is known that the release of sulfur-containing combustion products from power plant boiler combustion chambers or like industrial combustion systems, primarily as a result of the combustion of the high sulfur fuels utilized with increasing frequency, is detrimental to the environment and to the health of inhabitants of areas surrounding the power plant and may even adversely affect the environment at locations quite distal therefrom.
For this reason scrubbers have been developed for the desulfurization of such flue gases which can have a flue gas inlet, a flue gas outlet, means between the inlet and the outlet for treating the flue gas with a scrubbing solution which can contain one or more substances capable of binding the sulfur compounds, a sump below the washing zone for collecting the scrubbing liquid which can contain, in addition to the sulfur compounds, particulates scrubbed from the flue gas and other substances soluble in or absorbed by the scrubbing liquid and means for introducing oxygen into the sump and also for introducing additives to the sump.
The purpose of introducing oxygen into the sump is to oxidize any sulfite in the scrubbing liquid to sulfate, thereby allowing the recovery of calcium sulfate as a solid product (gypsum) for use in the construction industry.
The scrubbing liquid decanted from the precipitate solids may be recirculated and can include part of the soluble compounds formed as a product and which therefore may be prevented from sedimenting out. The additives which are fed to the scrubbing water can include lime, limestone and other calcium containing compounds as described in the aforementioned application which ensure that the sulfate when and if it is formed and when and if it is precipitated, will precipitate out as calcium sulfate.
To avoid the deposition of gypsum on the scrubber walls and baffles or other formations within the scrubber, the scrubbing liquid is operated with a calcium sulfate content of about 20 to 150 grams per liter, i.e. a relatively high concentration.
This high gypsum content ensures precipitation of the gypsum in the sump of the scrubber both during operation and when, for some reason, the system is brought to standstill, i.e. recirculation of the scrubbing liquid is terminated.
Conventional scrubbers are provided with numerous mechanical stirrers or agitators designed to prevent precipitation of the solids at various locations at which such precipitation is not desired.
This, of course, is disadvantageous since such stirrers and agitators require considerable maintenance, introduce significant downtime possibilities and create high capital, energy and other operating costs.
In addition, they generally require passages through the walls of the scrubber which must be sealed and the seals have maintenance and like problems.
Indeed, should one or more of the agitators somehow fail, the locations at which such agitators are provided generally are prone to relatively dense packing of precipitated solids which make it difficult, if not impossible, to restart the agitator, thereby aggravating the problem still further.
Finally, the operation of such scrubbers requires a standby source of emergency power to operate the agitators even when the main power supply fails either to keep the agitators working or to remove the liquid with its high gypsum content so that undesired deposit of solids does not occur.
The removal itself has the disadvantage that on restarting of the apparatus it is necessary to build up the concentration of gypsum again in the scrubbing liquid and hence the most desired mode of operation is one which retains the scrubbing liquid in the scrubber.